Information from manufacturer:

"With the abundance of lighting functions and wanting to see and check them before I placed the shell on the frame, I decided I needed a way to see what was happening! I went to work and prototyped a small connector board with LEDs so I could put the frame on the test track and test the motor, speaker and lighting functions before assembling the locomotive. The Light Test Board is also a good diagnostic tool to see if the decoder and motherboard are behaving properly! The Light Test Board works with the 8 output Original Decoder Buddy, and both the 12 output V5 and V5B."